What is the best way to trigger when a timestamp becomes old, i.e. more than 60 minutes ago?

I have a monitor that captures mobile devices (like laptops, phones, etc) and updates a timestamp when they were last seen. I would like to be able to trigger actions when that "last seen" value becomes an hour old, however the only triggers depend on a variable changing. In this case, it is almost the opposite problem: I need to trigger when the variable hasn't changes for an hour.

Thanks for the pointer to Fing... There are other cases where I use a sensor input to record temperature and such. I want to make sure those are updating and send me a notice when they are not updated in a given time.